Histamine, SRS-A (Slow Reacting Substance of Anaphylaxis), arginine esterase, plasminogen activator and prostaglandins are released from human lung during passive sensitization by the addition of specific antigen. The conditions necessary for their release and the role of these compounds in allergic asthma in man was studied. The data provide a possible explanation for the well known aspirin sensitivity seen in about 10 per cent of adult asthmatics.